In recent years, an illuminance sensor for detecting surrounding brightness is provided in a liquid crystal panel of an electronic apparatus such as a mobile phone or a digital camera in order to control a light emission amount of a backlight of the liquid crystal panel in accordance with illuminance of disturbance (light such as solar light or fluorescent light). The light emission control of the backlight is carried out in accordance with brightness perceived by a user of the electronic apparatus, i.e., a human. As such, it is important that a spectral characteristic (spectral sensitivity characteristic) of the illuminance sensor be close to visual sensitivity (visual sensitivity characteristic).
Generally, a spectral characteristic close to visual sensitivity is achieved in an illuminance sensor by a method of performing subtraction with respect to electric currents of a plurality of photodiodes having different spectral characteristics (see, for example, Patent Literatures 1, 2, etc.).
Patent Literature 1 discloses an optical sensor circuit in which a spectral characteristic close to visual sensitivity is achieved by performing subtraction with respect to electric currents flowing through respective two photodiodes having different spectral characteristics with the use of a current mirror circuit. FIG. 20 is a circuit diagram illustrating an outline configuration of the optical sensor circuit 900 disclosed in Patent Literature 1. As illustrated in FIG. 20, the optical sensor circuit 900 includes a photodiode PD901, a photodiode PD902, and transistors Tr901 and Tr902 constituting a current mirror circuit. The optical sensor circuit 900 has an output terminal OUT that is connected to a drain terminal of the transistor Tr902 and to a cathode terminal of the photodiode PD902. The photodiode PD901 has a spectral characteristic having sensitivity in a wavelength range of infrared rays (hereinafter abbreviated as “spectral characteristic of infrared rays”), and the photodiode PD902 has a spectral characteristic having sensitivity in a wavelength range from infrared rays to visible light (hereinafter abbreviated as “spectral characteristic from visible light to infrared rays”. Note that an electric current flowing when the photodiode PD901 receives light is referred to as an input electric current Iin901, and an electric current flowing when the photodiode PD902 receives light is referred to as an input electric current Iin902.
When the photodiodes PD901 and PD902 receive light, the input electric current Iin901 flows through the photodiode PD901, and this input electric current Iin901 flows also through the transistor Tr901. Since the transistor Tr901 constitutes the current mirror circuit together with the transistor Tr902, an electric current of (Iin901×α) flows through the transistor Tr902 (α: current mirror ratio).
Meanwhile, when the photodiodes PD901 and PD902 receive light, the input electric current Iin902 flows through the photodiode PD902. Accordingly, an electric current of (Iin902−Iin901×α) which is obtained by subtracting the electric current flowing through the transistor Tr902 from the input electric current Iin902 flows through the output terminal OUT. This electric current amount (Iin902−Iin901×α) allows a spectral characteristic close to visual sensitivity to be a chieved since sensitivity to wavelengths of infrared rays is reduced.
As described above, the sensor circuit 900 is capable of achieving a spectral characteristic close to visual sensitivity by performing subtraction with respect to the electric currents (the input electric currents Iin901 and Iin902) respectively flowing through the two photodiodes PD901 and PD902 having different spectral characteristics with the use of the current mirror circuit. Further, the photodiode PD901 is disposed so as to be sandwiched by the photodiode PD902. This reduces unevenness of an output that occurs due to an angle of light.
Patent Literature 2 discloses an illuminance sensor for achieving a spectral characteristic close to visual sensitivity by directly performing subtraction with respect to electric currents flowing through two respective photodiodes having different spectral characteristics. FIG. 21 is a plan view illustrating an outline configuration of a light receiving element 910 provided in the illuminance sensor disclosed in Patent Literature 2. As illustrated in FIG. 21, the light receiving element 910 includes a first light receiving section having a spectral characteristic in a range approximately from visible light to infrared rays and a second light receiving section having a spectral characteristic of approximately infrared rays. The first light receiving section is divided into two sections (first light receiving sections PDA) having almost the same area, and the first light receiving sections PDA thus divided are connected in parallel. The second light receiving section is divided into two sections (second light receiving sections PDB) having almost the same area, and the second light receiving sections PDB thus divided are connected in parallel. Accordingly, a spectral characteristic close to visual sensitivity can be achieved by subtracting an electric current flowing through the second light receiving section from an electric current flowing through the first light receiving section.
Further, a lens section (not illustrated) for focusing light onto the first light receiving sections PDA and the second light receiving sections PDB is provided above the light receiving element 910. The lens section focuses light onto a lens spot 911 of FIG. 21. The first light receiving sections PDA and the second light receiving sections PDB are alternately disposed within the lens spot 911. By thus distributing the first light receiving sections PDA and the second light receiving sections PDB uniformly in plan view within the lens spot 911, unevenness of an output which occurs due o an angle of light is reduced.
In recent years, an illuminance sensor is required to have high resolution. Accordingly, a digital-type illuminance sensor is becoming mainstream in replacement of a conventional analog-type illuminance sensor. A digital-type illuminance sensor generally includes an analog/digital converting circuit for converting an output into a digital value.
FIG. 22 is a circuit diagram illustrating an outline configuration a digital-type illuminance sensor 920. As illustrated in FIG. 22, the illuminance sensor 920 includes a photodiode PD921 having a spectral characteristic of infrared rays, a photodiode PD922 having a spectral characteristic from visible light to infrared rays, analog-digital converting circuits (hereinafter abbreviated as “AD converting circuits”) ADC921 and ADC922, a multiplying section 923, and a subtracting section 924. Note that an electric current flowing when the photodiode PD921 receives light is referred to as an input electric current Iin921, and an electric current flowing when the photodiode PD922 receives light is referred to as an input electric current Iin922.
When the photodiodes PD921 and PD922 receive light, the input electric current Iin921 flows through the photodiode PD921 and this input electric current Iin921 is supplied to the AD converting circuit ADC921, and the input electric current Iin922 flows through the photodiode PD922 and this input electric current Iin922 is supplied to the AD converting circuit ADC922.
The AD converting circuit ADC921 converts the input electric current Iin921 into a digital value, and outputs the digital value as a measurement signal ADCOUT921. The measurement signal ADCOUT921 is multiplied by α (α: constant value) by the multiplying section 923, and is then supplied to the subtracting section 924. The AD converting circuit ADC922 converts the input electric current Iin922 into a digital value, and outputs the digital value as a measurement signal ADCOUT922. The measurement signal ADCOUT922 is supplied to the subtracting section 924.
The subtracting section 924 subtracts the measurement signal ADCOUT921 multiplied by α from the measurement signal ADCOUT922. The subtracting section 924 thus outputs a measurement signal of (ADCOUT922−ADCOUT921×α). This measurement signal (ADCOUT922−ADCOUT921×α) allows a spectral characteristic close to visual sensitivity to be achieved. That is, the same result as that obtained by the optical sensor circuit 900 of FIG. 20 can be obtained by digital computation (“ADCOUT922−ADCOUT921×α=lin902−Iin901×α”).
As described above, the illuminance sensor 920 can achieve a spectral characteristic close to visual sensitivity by converting the electric currents (the input electric currents Iin921 and Iin922) respectively flowing through the two photodiodes PD921 and PD922 having different spectral characteristics into digital values (the measurement signals ADCOUT921 and ADCOUT922) and then performing subtraction. Further, in a case where a light detection result (measured illuminance value) is outputted as a digital signal as in the illuminance sensor 920, processing using software in a CPU or a microcomputer becomes easy in a later step using the digital signal.
The AD converting circuits ADC921 and ADC922 are not limited to a specific configuration, but generally have an integral configuration. This is because an integral AD converting circuit allows highly accurate resolution with a simple configuration, and is therefore suitable for a device, such as an illuminance sensor, for which slow but high resolution (approximately 16 bit) is required.